12,128 research outputs found
PT symmetry and large-N models
Recently developed methods for PT-symmetric models can be applied to
quantum-mechanical matrix and vector models. In matrix models, the calculation
of all singlet wave functions can be reduced to the solution a one-dimensional
PT-symmetric model. The large-N limit of a wide class of matrix models exists,
and properties of the lowest-lying singlet state can be computed using WKB. For
models with cubic and quartic interactions, the ground state energy appears to
show rapid convergence to the large-N limit. For the special case of a quartic
model, we find explicitly an isospectral Hermitian matrix model. The Hermitian
form for a vector model with O(N) symmetry can also be found, and shows many
unusual features. The effective potential obtained in the large-N limit of the
Hermitian form is shown to be identical to the form obtained from the original
PT-symmetric model using familiar constraint field methods. The analogous
constraint field prescription in four dimensions suggests that PT-symmetric
scalar field theories are asymptotically free.Comment: 15 pages, to be published in J. Phys. A special issue on Pseudo
Hermitian Hamiltonians in Quantum Physic
Addictive links: The motivational value of adaptive link annotation
Adaptive link annotation is a popular adaptive navigation support technology. Empirical studies of adaptive annotation in the educational context have demonstrated that it can help students to acquire knowledge faster, improve learning outcomes, reduce navigational overhead, and encourage non-sequential navigation. In this paper, we present our exploration of a lesser known effect of adaptive annotation, its ability to significantly increase students' motivation to work with non-mandatory educational content. We explored this effect and confirmed its significance in the context of two different adaptive hypermedia systems. The paper presents and discusses the results of our work
Inner Molecular Rings in Barred Galaxies: BIMA SONG CO Observations
Although inner star-forming rings are common in optical images of barred
spiral galaxies, observational evidence for the accompanying molecular gas has
been scarce. In this paper we present images of molecular inner rings, traced
using the CO (1-0) emission line, from the
Berkeley-Illinois-Maryland-Association Survey of Nearby Galaxies (BIMA SONG).
We detect inner ring CO emission from all five SONG barred galaxies classified
as inner ring (type (r)). We also examine the seven SONG barred galaxies
classified as inner spiral (type (s)); in one of these, NGC 3627, we find
morphological and kinematic evidence for a molecular inner ring. Inner ring
galaxies have been classified as such based on optical images, which emphasize
recent star formation. We consider the possibility that there may exist inner
rings in which star formation efficiency is not enhanced. However, we find that
in NGC 3627 the inner ring star formation efficiency is enhanced relative to
most other regions in that galaxy. We note that the SONG (r) galaxies have a
paucity of CO and H alpha emission interior to the inner ring (except near the
nucleus), while NGC 3627 has relatively bright bar CO and H alpha emission; we
suggest that galaxies with inner rings such as NGC 3627 may be misclassified if
there are significant amounts of gas and star formation in the bar.Comment: To be published in the Astrophysical Journal, July 2002 A version of
the paper with full resolution figures is available at:
http://www.astro.umd.edu/~mregan/ms.ps.g
The Mass Inflow Rate in the Barred Galaxy NGC 1530
Mass inflow in barred galaxies has been invoked to account for a wide variety
of phenomena, but until now direct evidence for inflow has been lacking. We
present Fabry-Perot H-alpha observations of the barred spiral galaxy NGC 1530
from which we determine velocities of the ionized gas for the entire region
swept by the bar. We compare the velocity field to models of gas flow in barred
spirals and show that it is well reproduced by ideal gas hydrodynamic models.
Inspection of the models and observations reveals that gas entering the bar
dust lanes streams directly down the dust lanes toward the 2 kpc radius nuclear
ring. The models predict that approximately 20% of the gas flowing down the
dust lane enters the nuclear ring; the remaining gas sprays around the ring to
the other bar dust lane. The fraction of the gas entering the ring is
relatively insensitive to the shape or size of the bar. Our observations of the
velocity field and dust optical depth yield a mass inflow rate into the nuclear
ring of 1 solar mass per year.Comment: 13 pages, 3 figures, aastex 4.0, accepted for publication in Ap J
Letter
Improved test of Lorentz Invariance in Electrodynamics using Rotating Cryogenic Sapphire Oscillators
We present new results from our test of Lorentz invariance, which compares
two orthogonal cryogenic sapphire microwave oscillators rotating in the lab. We
have now acquired over 1 year of data, allowing us to avoid the short data set
approximation (less than 1 year) that assumes no cancelation occurs between the
and parameters from the photon
sector of the standard model extension. Thus, we are able to place independent
limits on all eight and parameters.
Our results represents up to a factor of 10 improvement over previous non
rotating measurements (which independently constrained 7 parameters), and is a
slight improvement (except for ) over results from
previous rotating experiments that assumed the short data set approximation.
Also, an analysis in the Robertson-Mansouri-Sexl framework allows us to place a
new limit on the isotropy parameter of
, an improvement of a factor of 2.Comment: Accepted for publication in Phys. Rev.
Recommended from our members
The Seasonal Evolution of Albedo across Glaciers and the Surrounding Landscape of Taylor Valley, Antarctica
The McMurdo Dry Valleys (MDVs) of Antarctica are a polar desert ecosystem consisting of alpine glaciers, ice-covered lakes, streams, and expanses of vegetation-free rocky soil. Because average summer temperatures are close to 0 Cel., the MDV ecosystem in general, and glacier melt dynamics in particular, are both closely linked to the energy balance. A slight increase in incoming radiation or change in albedo can have large effects on the timing and volume of meltwater. However, the seasonal evolution or spatial variability of albedo in the valleys has yet to fully characterized. In this study, we aim to understand the drivers of landscape albedo change within and across seasons. To do so, a box with a camera, GPS, and shortwave radiometer was hung from a helicopter that flew transects four to five times a season along Taylor Valley. Measurements were repeated over three seasons. These data were coupled with incoming radiation measured at six meteorological stations distributed along the valley to calculate the distribution of albedo across individual glaciers, lakes, and soil surfaces. We hypothesized that albedo would decrease throughout the austral summer with ablation of snow patches and increasing sediment exposure on the glacier and lake surfaces. However, small snow events (\u3c 6mm water equivalent) coupled with ice whitening caused spatial and temporal variability of albedo across the entire landscape. Glaciers frequently followed a pattern of increasing albedo with increasing elevation, as well as increasing albedo moving from east to west laterally across the ablation zone. We suggest that spatial patterns of albedo are a function of landscape morphology trapping snow and sediment, longitudinal gradients in snowfall magnitude, and wind-driven snow redistribution from east to west along the valley. We also compare our albedo measurements to the MODIS albedo product and found that overall the data have reasonable agreement. The mismatch in spatial scale between these two datasets results in variability, which is reduced after a snow event due to albedo following valley-scale gradients of snowfall magnitude. These findings highlight the importance of understanding the spatial and temporal variability in albedo and the close coupling of climate and landscape response. This new understanding of landscape albedo can constrain landscape energy budgets, better predict meltwater generation on from MDV glaciers, and how these ecosystems will respond to changing climate at the landscape scale
Equilibration of isolated macroscopic quantum systems
We investigate the equilibration of an isolated macroscopic quantum system in
the sense that deviations from a steady state become unmeasurably small for the
overwhelming majority of times within any sufficiently large time interval. The
main requirements are that the initial state, possibly far from equilibrium,
exhibits a macroscopic population of at most one energy level and that
degeneracies of energy eigenvalues and of energy gaps (differences of energy
eigenvalues) are not of exceedingly large multiplicities. Our approach closely
follows and extends recent works by Short and Farrelly [2012 New J. Phys. 14
013063], in particular going beyond the realm of finite-dimensional systems and
large effective dimensions.Comment: 19 page
Identity dynamics as a barrier to organizational change
This article seeks to explore the construction of group and professional identities in situations of organizational change. It considers empirical material drawn from a health demonstration project funded by the Scottish Executive Health Department, and uses insights from this project to discuss issues that arise from identity construction(s) and organizational change. In the course of the project studied here, a new organizational form was developed which involved a network arrangement with a voluntary sector organization and the employment of âlay-workersâ in what had traditionally been a professional setting. Our analysis of the way actors made sense of their identities reveals that characterizations of both self and other became barriers to the change process. These identity dynamics were significant in determining the way people interpreted and responded to change within this project and which may relate to other change-oriented situations
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